Dynamic completion fails on Fish for Windows/MSYS2

[Nahor]

Environment: Windows MSYS2, with package mingw-w64-ucrt-x86_64-jj 0.36.0-1

Completion on Fish under MSYS2 fails because the path to jj.exe is invalid:

$ jj<tab>
fish: Unknown command: 'C:\\msys64\\ucrt64\\bin\\jj.exe'
fish:
COMPLETE=fish "C:\msys64\ucrt64\bin\jj.exe" -- (commandline --current-process --tokenize --cut-at-cursor) (commandline --current-token)
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~^
in command substitution

(slightly approximate output since things are a bit messed and hard to read otherwise)

Also:

$ COMPLETE=fish jj
complete --keep-order --exclusive --command jj --arguments "(COMPLETE=fish "'"C:\\msys64\\ucrt64\\bin\\jj.exe"'" -- (commandline --current-process --tokenize --cut-at-cursor) (commandline --current-token))"

In both cases, note the C:\msys64\ucrt64\bin\jj.exe, which is is not a valid path under Cygwin or MSYS2.

Using c:/msys64/ucrt64/bin/jj.exe (i.e. forward slashes instead of back slashes) works:

$ COMPLETE=fish jj | sed 's#\\\\\\\\#/#g' | source
$ jj c<tab>
commit  (Update the description and create a new change on top [default alias: ci])  config  (Manage config options
$ jj co

More generally, Windows accepts forward slashes as a path separator, including in Git-for-Windows Bash. So it should be safe to use that separator in jj unconditionally, it should work in all Windows environments.

Issue #7024 replaced some of those backslashes but seems to have missed this completion.